Questions
5–8 MCQs per paper
Difficulty
Medium-Hard
Importance
High yield for JEE Main/Advanced and NEET
Overview
Hydrocarbons are the foundational pillar of organic chemistry, forming the skeleton for all complex functional group derivatives. Mastering this unit is essential for scoring in JEE and NEET, as it serves as the basis for understanding reaction mechanisms, stereochemistry, and synthetic pathways.
Alkanes: Substitution and Combustion
Alkanes are chemically inert but undergo free-radical substitution. The focus in competitive exams is on the stability of free-radical intermediates and halogenation patterns.
- Reactivity order of halogens: F2 > Cl2 > Br2 > I2
- Stability of radicals: 3 > 2 > 1 > methyl
- Wurtz reaction for symmetrical alkane synthesis
- Corey-House synthesis for unsymmetrical alkanes
- Decarboxylation using Soda Lime (NaOH + CaO)
Alkenes: Electrophilic Addition
Alkenes are electron-rich, making them prone to electrophilic addition. You must master the regioselectivity governed by carbocation stability and rearrangement potential.
- Markovnikov's rule: H adds to carbon with more H
- Anti-Markovnikov: Peroxide effect (only for HBr)
- Ozonolysis: Cleavage into aldehydes/ketones
- Hydroboration-Oxidation: Syn-addition, anti-Markovnikov alcohol
- Oxymercuration-Demercuration: Markovnikov alcohol without rearrangement
Alkynes: Acidity and Nucleophilicity
Terminal alkynes exhibit acidic behavior due to the high s-character of sp-hybridized carbons. Electrophilic addition here follows the same rules as alkenes but usually stops at the alkene stage unless excess reagent is used.
- Terminal alkynes react with NaNH2/NaNH3
- Lindlar catalyst produces cis-alkene
- Na/Li in liquid NH3 (Birch reduction) produces trans-alkene
- Hydration: HgSO4/H2SO4 leads to keto-enol tautomerism
- Nucleophilic character allows carbon-carbon bond formation
Aromatic Compounds: Electrophilic Substitution
Aromaticity is defined by Huckel's rule. The core chemistry involves Electrophilic Aromatic Substitution (EAS), where substituents dictate the position of subsequent attacks.
- Huckel's rule: (4n + 2) pi electrons
- Ortho/Para directing: -OH, -NH2, -R, -X
- Meta directing: -NO2, -CHO, -COOH, -SO3H
- Friedel-Crafts alkylation/acylation
- Activation vs. Deactivation groups
Formula Sheet
Huckel Rule: 4n + 2
Wurtz: R-X + 2Na -> R-R
Friedel-Crafts: Ar-H + R-Cl -> Ar-R + HCl
Exam Tip
Always check for carbocation stability and potential rearrangements before finalizing the product of an electrophilic addition reaction.
Common Mistakes
- Ignoring carbocation rearrangements during electrophilic addition reactions
- Confusing the stereochemical outcomes of syn vs anti-addition (e.g., Baeyer's reagent vs anti-dihydroxylation)
- Applying peroxide effect to HCl or HI instead of just HBr
More Revision Notes
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